The majority of the energy used today is derived from fossil fuels, despite the on-going controversy surrounding their environmental impact. The extraction of fossil fuels for energy production results in the release of carbon into the atmosphere that was previously stored in the earth, and thereby has a net effect of increasing the levels of atmospheric CO2. On the other hand, CO2 released from combusting cellulosic feedstock is relatively benign, given that it simply returns to the atmosphere carbon that was previously removed therefrom by plant photosynthesis. Displacing fossil-based fuel with biomass derived fuel creates greenhouse gas (GHG) benefits by displacing CO2 emissions that would have been from the fossil fuel. Various governments have promoted the increased use of renewable fuel through legislative and regulatory regimes, including the Energy Independence and Security Act (EISA) in the United States.
Although the use of cellulosic feedstocks to produce biofuels is known to have the potential to reduce GHG emissions from the fossil fuel industry, this has proven to be difficult to achieve in practice due to economic and technical challenges. Despite much effort and research on producing energy from renewable energy sources, non-fossil fuel options are still in their infancy. Thus, there is a need in the art for new sources of energy that are more environmentally benign than fossil fuels and that overcome some of the economic and technical challenges inherent in known processes for utilizing the energy from cellulosic feedstock.
Biogas is a biofuel produced by anaerobic digestion that has been receiving increased attention in recent years. Biogas contains predominantly methane and is commonly used as heating fuel or for electricity generation, although it can potentially also be used as a transportation fuel or as an intermediate to produce another fuel. A benefit of making biogas is that a wide variety of feedstocks can be used to produce the gas, including landfill waste or waste streams from commercial plants. In the production of ethanol from cellulosic feedstocks, waste streams remaining after the recovery of ethanol are often treated by anaerobic digestion to produce biogas. Landfills also produce biogas through anaerobic digestion of municipal waste. While the biogas is commonly used on-site for heating or electricity generation, for example at a plant or a landfill site, its use is less widespread in the transportation sector. However, commercializing the use of biogas in other applications besides on-site use at a plant or other facility would be desirable.
Biofuel can also be produced from cellulosic feedstock by thermal processes such as gasification. Gasification includes processes in which cellulosic feedstock is subjected to high temperatures to make syngas comprising hydrogen and carbon monoxide and optionally other components such as carbon dioxide, methane and water. The gasification step to produce syngas is generally carried out above 500° C. up to 1500° C. The syngas in turn can be used as an intermediate to produce other fuels. For example, to generate methane from the syngas, a methanation reaction of syngas is conducted to produce a fuel comprised primarily of methane. Other thermal processing of cellulosic feedstock, such as pyrolysis and combustion, can be used to produce biofuels such as pyrolysis oil or other energy products, including electricity and heat as described herein.
However, the thermal processing of streams from cellulosic feedstock, including combustion, gasification and/or pyrolysis, can be hindered by the presence of various extractives present in cellulosic feedstock. Such extractives include inorganic salt, silica, pentose sugars, hexose sugars and/or organic acids, which may produce slag, pollutants and other undesirable components during thermal processing. Inorganic salt can be particularly problematic as it can form a low melting material produced by the combination of silica with alkali salts. This low melting point material can lead to fouling of boilers or gasification units, requiring shut-downs and expensive cleaning. Extractives can also contribute to problematic tars in gasification operations.
There is thus a need in the art to overcome some of the challenges of making fuels, fuel intermediates and/or energy products from cellulosic feedstock, particularly biogas and products from thermal processes. A process for improving the efficiency of such processes while maintaining a beneficial GHG emission impact could meet this need.